In control of a mobile unit such as a robot, it is important to accurately estimate a self-position. It has been known that an omnidirectional camera, a fisheye lens camera or the like is mounted on the mobile unit as a visual sensor, and a map of a landmark to be seen by the camera is stored, and that the self-position is estimated using an image from the camera with reference to the map. For example, in Non-Patent Document 1, three candidates for the landmarks are randomly extracted from the camera image, and three landmarks are randomly extracted from the map to associate these on a one-on-one basis, thereby setting a candidate for a self-position. Next, assuming that this candidate position is correct, an angle of visibility from the estimated self-position with respect to another landmark of the map is found to find an error between itself and an angle of visibility with respect to a candidate for the another landmark in the camera image. The association between the candidate for the landmark and the map in which the above-described error is minimum is determined to be correct, and a self-position based on this is outputted. For evaluation of the error, for example, an error in angle of visibility between the candidate for the landmark in the camera image and the landmark on the map is squared to use a median value thereof. Not only the median value but also an integrated value of the error or the like may be used.
However, in an artificial circumstance, such as inside a building, bringing in another article may hinder the landmark on the map from being seen, bringing in an article confusing the landmark or lighting may make the landmark less visible, and so on, after creating the map. By these, the landmark on the map may disappear, or other than the candidate for the landmark, in other words, a false landmark may be extracted. The disappearance of the landmark or the false landmark drastically makes the estimation of the self-position complicated. Consequently, a mark that can be surely recognized from the camera image and does not confuse another article needs to be searched. In this respect, Patent Document 1 (Japanese Laid-Open Patent Publication No. 2004-34272) proposes that a fluorescent lamp on a ceiling is set as a landmark. However, when in a large room, there are a number of fluorescent lamps, all the fluorescent lamps have the same shape and are arranged regularly, and thus, it is not easy to recognize which of the fluorescent lamps is seen in the camera image.    Non-Patent Document 1: Robust Self-Position Identification Method of Soccer Robot Having Omnidirectional Camera and Dead Reckoning Function, Journal of the Robotics Society of Japan Vol. 22, No. 3, pp. 343-352, 2004, 4.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-34272